Recently, an SOI wafer having SOI structure in which a silicon layer (an SOI layer) is formed on an insulator has been especially attracting attention as a wafer for high-performance LSI for an electronic device because the SOI wafer is excellent in high-speed property, low power consumption, high breakdown voltage, environmental resistance, etc. of the device.
Representative production methods of the SOI wafer are bonding method, SIMOX (Separation by ion-implanted oxygen) method, and so on. The bonding method is a method of producing an SOI wafer by forming an oxide film on at least one of a bond wafer to form an SOI layer and a base wafer to be a supporting substrate, bonding the bond wafer to the base wafer through the oxide film, and making the bond wafer into a thin film.
Also, the SIMOX method is a method of producing an SOI wafer (SIMOX wafer) by subjecting to heat treatment to form an oxide film in a silicon wafer after implanting oxygen ions into the silicon wafer. More concretely, as shown in FIG. 2 for example, first a silicon wafer 11 subjected to mirror polishing etc. is prepared (Step (a′)), and sequentially an oxygen ion-implanted layer 12 is formed by implanting oxygen ions (O+) into the silicon wafer 11 heated at a temperature of about 500° C. from one main surface thereof at Step (b′). In this case, as an ion implantation condition, implantation energy is set generally 150-200 keV, and dose amount of oxygen ions is required to be high dose amount, about 1.5×1018/cm2 or more, to form a continuous buried oxide film.
And thereafter, at a Step (c′), oxide film-forming heat treatment to convert the oxygen ion-implanted layer 12 formed in the wafer into a buried oxide film 13 is performed, for example, in an atmosphere of an inert gas at a temperature of 1300° C. or more. Thereby SOI wafer 15 in which an SOI layer 14 is formed on the buried oxide film 13 can be produced.
The SOI wafer produced by SIMOX method as described above has advantages that there can be easily obtained excellent film thickness uniformity because a film thickness of the SOI layer and the buried oxide film can be determined by the ion implantation energy and dose amount when performing the oxygen ion implantation. And also the SOI wafer can be produced from one silicon wafer without requiring two wafers as the above bonding method. Therefore the SOI wafer can be produced at comparatively low cost.
However, in the case of performing the oxygen ion implantation by the high dose amount as above by using SIMOX method, although perfectness of the buried oxide film can be enhanced, there is an problem that it is difficult to obtain an SOI layer having a good crystal quality because damage is introduced to the wafer surface transmitted by a large amount of the oxygen ions so that threading dislocation is easily generated at high density in the SOI layer when performing the oxide film-forming heat treatment.
To solve such problems, various studies and investigations have been repeated. Through them, it was found that a continuous buried oxide film could be formed by oxygen ion implantation even at low dose amount. Thereby, an SOI wafer of which density of threading dislocation is low came to be produced (See “Science of SOI”, edited by UCS Semiconductor Substrate Technology Workshop, published by Realize publishers, pp. 26-30). In this case, because the dose amount of the oxygen ions is limited to about 4×1017/cm2, the range of the dose amount is known as a dose window.
Though threading dislocation in the buried oxide film formed by such low dose amount can be reduced, pin holes causing insulation failure of the buried oxide film are easily formed. Therefore a quality of the buried oxide film is lower than that of the buried oxide film formed by high dose amount.
Accordingly, various methods are suggested to improve the quality of the buried oxide film formed by ion implantation by such low dose amount. For example a method was suggested that the quality of the buried oxide film is improved by so-called Internal Thermal Oxidation treatment (it is also described, “ITOX treatment” hereafter) in which after general oxide film-forming heat treatment, an oxidation treatment in the oxygen atmosphere at high temperature is sequentially performed (see, Publication of Japanese Patent No. 3036619). Thus, by adding ITOX treatment to SIMOX method performing oxygen ion implantation by such low dose amount, the buried oxide film is thickened so that the quality thereof is improved. Therefore it has become possible to produce a SIMOX wafer with high quality in which density of pin holes is low and perfectness of the buried oxide film is enhanced.
However, with higher integration of semiconductor devices in these years, production of a higher-quality SOI wafer is desired and, for example, an SOI wafer having a buried oxide film with a thin thickness is required. Moreover, it is believed that, in future, a thickness of the buried oxide film of an SOI wafer would become even thinner from 100 nm to 50 nm or less.
However, when an SOI wafer is produced by using SIMOX method as above, for example if oxygen ions are implanted by high dose amount required to form the continuous buried oxide film, a thickness of a buried oxide film formed an SOI wafer always become thicker than one thickness, and it was impossible to form the buried oxide film with a thin thickness as described above.
Also, when an SOI wafer is produced by implanting oxygen ions by low dose amount, about 4×1017/cm2 (dose window), the buried oxide film with thin thickness can be formed. However, because a quality of the buried oxide film is low, it is necessary to perform ITOX treatment to improve the quality of the buried oxide film as described above, and the buried oxide film in the silicon wafer is thickened to be thick one. Accordingly, it was extremely difficult to produce, by SIMOX method, an SOI wafer having the buried oxide film of which perfectness is high and the film thickness is thin as required in these days.
Moreover, an SOI wafer produced by the above SIMOX method is excellent in film thickness uniformity and production cost, compared to an SOI wafer produced by the aforementioned bonding method. However, there are problems that crystal defects existing in the SOI layer are much and crystallinity of the SOI layer is inferior and also surface roughness of the SOI layer is large. Therefore, in production of an SOI wafer by SIMOX method, improvement of crystallinity of an SOI layer and improvement of surface quality are desired.